Optical tags, for example bar codes, are well known. They are widely used for labeling consumer products in retailing premises such a supermarkets. Moreover, they have received widespread acceptance on account of their low cost and the ease with which they can be interrogated using scanned laser beams. In many cases, such bar codes are of printed form and an integral part of product package labeling, for example on paper sleeves around packaged food items. Alternatively, in other cases, the optical tags are retrospectively added to package labels by way of non-contact ink-jet printing.
Optical tags have the disadvantage that they are often susceptible to being copied with relative ease. Such copying can render it difficult to determine the authenticity of counterfeit merchandise bearing similar optical bar codes to corresponding genuine merchandise. In order to address this difficulty, some types of optical tag are arranged to be a covert part of existing labels or product graphical design detail, for example as in documents of value such as bank notes or bills of exchange. For example, it is known to incorporate optical tag features as part of a picture on labels, the picture appearing normal to the human eye and the optical tag features being subtly incorporated so that they are not discernible on cursory human visual inspection.
The inventors of the present invention have appreciated that there exists a need for optical tags which are more difficult to copy and provide a higher degree of authenticity validation to products onto which they are affixed or are an integral part.